Differential host responses to parasitism shape divergent fitness costs of infection

Fitness costs of infection are fundamental to understanding the ecology and evolution of host-parasite interactions. However, these costs, and particularly their underlying mechanisms, are challenging to evaluate in wild populations. Here, we quantified total and species-specific costs of gastrointestinal worms on African buffalo, by combining the power of an anthelmintic treatment experiment that perturbed the entire worm community with a longitudinal study that tracked the two most dominant community members. Reducing all worms improved buffalo body condition, which was strongly associated with increases in survival and reproduction. Species-specific analyses revealed that condition-mediated fitness costs of infection differed between parasite species. Hosts that gained the blood-sucking worm Haemonchus, lost condition, and this loss may have been mediated by reductions in forage intake. Hosts that resisted Haemonchus by elevating IL-4 and eosinophil immune defences were able to reduc...